Vehicle speed control system and method

ABSTRACT

Embodiments of the present invention provide a vehicle control system comprising a plurality of speed control systems each operable to cause the vehicle to operate in accordance with a respective target speed, the system being operable wherein one at said plurality of speed control systems may be selected to control vehicle speed at a given moment in time, wherein when responsibility for speed control is transferred from a first one of the plurality of speed control systems to a second one of the speed control systems, the second one of the speed control systems is operable to set a value of target speed thereof to a value corresponding to that of the target speed of the first.

FIELD OF THE INVENTION

The invention relates to a system for controlling the speed of avehicle. In particular, foul not exclusively, the invention relates to asystem for controlling the speed of a land-based vehicle which iscapable of driving in a variety of different and extreme terrains andconditions.

BACKGROUND TO THE INVENTION

In known vehicle speed control systems, typically referred to as cruisecontrol systems, the vehicle speed is maintained on-road once set by theuser without further intervention by the user so as to improve thedriving experience for the user by reducing workload.

The user selects a speed at which the vehicle is to be maintained, andthe vehicle is maintained at that speed for as long as the user does notapply a brake or, in some systems, the clutch. The cruise control systemtakes its speed signal from the driveshaft or wheel speed sensors. Whenthe brake or the clutch is depressed, the cruise control system isdisabled so that the user can override the cruise control system tochange the vehicle speed without resistance from the system. If the userdepresses the accelerator pedal the vehicle speed will increase, butonce the user removes his foot from the accelerator pedal the vehiclereverts to the pre-set cruise speed.

More sophisticated cruise control systems are integrated info the enginemanagement system and may include an adaptive functionality which takesinto account the distance to the vehicle in front using a radar-basedsystem. For example, the vehicle may be provided with a forward-lookingradar detection system so that the speed and distance of the vehicle infront is detected and a safe following speed and distance is maintainedautomatically without the need for user input. If the lead vehicle slowsdown, or another object is detected by the radar defection system, thesystem sends a signal to the engine or the braking system to slew thevehicle down accordingly, to maintain a safe following distance.

Such systems are usually operable only above a certain speed, typicallyaround 15-20 kph, and are ideal in circumstances in which the vehicle istravelling in steady traffic conditions, and particularly en highways ormotorways. In congested traffic conditions, however, where vehicle speedtends to vary widely, cruise control systems are ineffective, andespecially where the systems are inoperable because of a minimum speedrequirement. A minimum speed requirement is often imposed on cruisecontrol systems so as to reduce the likelihood of low speed collision,for example when parking. Such systems are therefore ineffective incertain driving conditions (e.g. low speed) and are set to beautomatically disabled in circumstances in which a user may not considerit to fee desirable to do so.

Known cruise control systems also cancel in the event that a wheel slipevent is detected requiring intervention by a traction control system(TCS) or stability control system (SCS). Accordingly, they are not wellsuited to maintaining vehicle progress when driving in off roadconditions where such events may be relatively common.

It is also known to provide a control system tor a motor vehicle forcontrolling one or more vehicle subsystems. U.S. Pat. No. 7,349,776, thecontent of which is hereby incorporated by reference, discloses avehicle control system comprising a plurality of subsystem controllersincluding an engine management system, a transmission controller, asteering controller, a brakes controller and a suspension controller.The subsystem controllers are each operable in a plurality of subsystemfunction modes. The subsystem controllers are connected to a vehiclemode controller which controls the subsystem controllers to assume arequired function mode so as to provide a number of driving modes, forthe vehicle. Each of the driving modes corresponds to a particulardriving condition or set of driving conditions, and in each mode each ofthe sub-systems is set to the function mode most appropriate to thoseconditions. Such conditions ere linked to types of terrain over whichthe vehicle may be driven such as grass/gravel/snow, mud and ruts, rockcrawl, sand and a highway mode known as ‘special programs off’ (SPO).The vehicle mode controller may be referred to as a Terrain Response(TR) (RTM) System or controller. The driving modes may also be referredto as terrain modes, terrain response modes, or control modes.

It is desirable to provide a speed control system capable of controllingvehicle speed at relatively low speeds and whilst driving in off roadconditions.

STATEMENTS OF INVENTION

Embodiments of the invention may be understood with reference to theappended claims.

Aspects of the present invention provide a system, a vehicle and amethod.

In one aspect of the invention for which protection is sought there isprovided a vehicle control system comprising a plurality of speedcontrol systems each operable to cause the vehicle to operate inaccordance with a respective target speed, wherein when responsibilityfor speed control is transferred from a first one of the plurality ofspeed control systems to a second one of the speed control systems, avalue of target speed of the second speed control system is set to avalue dependent on the target speed of the first speed control system.

Embodiments of the present invention have the advantage that the targetspeed of the second one of the speed control systems may be setindependently of the current vehicle speed. Thus, if when the drivercommands a change from one speed control system to another the vehicleis temporarily travelling at a speed that is greater than or less thanthe current value of target speed of the first one of the speed controlsystems, the value of target speed of the second one of the speedcontrol systems may be set to a value that is dependent on the value oftarget speed of the first speed control system, and not on the instantvehicle speed.

It is to be understood that if the newly selected speed control systemwere to set the value of target speed thereof to the instant vehiclespeed at the moment the new speed control system was selected, vehiclecomposure may be affected and driver workload increased. By way ofexample, when operating with the first one of the speed control systemsactive, the first speed control system may reduce temporarily the valueof target speed in dependence on one or more factors such as the terrainover which the vehicle is travelling. The first speed control system maysubsequently accelerate the vehicle back to the target speed value whenit determines that the terrain is suitable for travel at that value oftarget speed. Whilst the first one of the speed control systems isaccelerating the vehicle to the current target speed value, the drivermay choose to switch speed control to the second one of the speedcontrol systems from the first. If the second speed control system wereto set the value of target speed thereof to the instant vehicle speed,acceleration to the target speed value of the first speed control systemwould be interrupted, potentially reducing vehicle composure. If thedriver did wish to travel at the same value of target speed as the firstspeed control system, the driver would then be required to adjust thetarget speed of the second speed control system to the value of thefirst speed control system. Thus driver workload may be reduced inembodiments of the present invention.

Advantageously, the system may be operable to set the target speed ofthe second one of the speed control systems to the instant value of thetarget speed of the first one of the speed control systems when thesecond one of the speed control systems assumes responsibility forcontrolling vehicle speed from the first one of the speed controlsystems.

This feature has the advantage that an abrupt change in target speedwhen the second one of the speed control systems assumes responsibilityfor speed control from the first one of the speed control systems may beavoided. This may improve vehicle composure by preventing sudden changesin vehicle speed and allowing traction to be maintained on difficult andvaried terrain.

For example, a vehicle may be equipped with a low speed progress control(LSP) system (or low speed cruise control system) and a hill descentcontrol (HOC) system each operable to cause the vehicle to operate inaccordance with a target speed. If the LSP system is currently selectedand the target speed is set to 10 kph, if the driver subsequentlyselects a hill descent control (HDC) system to assume speed control theHDC system controls vehicle speed in accordance with a target speedhaving the same value, i.e. 10 kph. A driver may therefore enjoy asmooth, coordinated transition from one speed control system to another.

Suitable HDC systems are described in patent applications GB2342969,GB2341430, GB2325716 & GB2308415, the contents -of which areincorporated herein by reference.

One or more of the speed control systems may be operable to cause thevehicle to operate in accordance with a respective temporary maximumspeed value that is less than the instant target speed in dependence onone or more parameters.

Thus, if the temporary maximum speed value is less than the respectivetarget speed the selected speed control system causes the vehicle tooperate in accordance with the temporary maximum value and not thetarget speed.

Optionally, one or more of the speed control systems are operable toresume operation at the target speed thereof if the temporary maximumspeed value becomes greater than or equal to the target speed or thespeed control system no longer causes the vehicle to operate inaccordance with the temporary maximum speed value.

Advantageously the system may be operable to set the temporary maximumspeed value of one or more of the speed control systems in dependence ona type of terrain in which the vehicle is operating.

The system may be operable to receive one or more signals indicative ofa type of terrain in which the vehicle is operating whereby the systemmay set the maximum allowable value of target speed.

The one or more signals may be received by one of more of the speedcontrol systems, which in turn determine the maximum allowable value oftarget speed. Alternatively, the one or more signals may be received bya controller configured to set the value of maximum target speed of oneor more of the speed control systems In dependence on the type ofterrain.

The signals may include signals that are set in dependence on one ormore of ambient temperature, surface roughness of terrain in which thevehicle is operating, surface wetness or moisture state, surfacecoefficient of friction between a wheel and the terrain or any othersuitable factor that may influence choice of maximum allowable targetspeed.

The system may be operable to store data in respect of a previous valueof target speed for one or more of the speed control systems, wherein ifone said one or more speed control systems is reselected to beresponsible for speed control following deselection thereof, thereselected system is operable to resume operation at the value of targetspeed employed by the reselected system when the system was last

This feature has the advantage that a driver may conveniently resumeoperation at a previous target speed employed by a given speed controlsystem when speed control is resumed by that speed control system.

Thus, if a driver switches speed control systems from a first system toa second system, and reduces a target speed of the second system belowthe target speed set whilst the first system was operating, uponswitching back to the first system the driver may be given the option ofresuming operation at the previously set value of target speed of thefirst system, i.e. the value set prior to deselecting the first speedcontrol system.

The system may be configured wherein if responsibility for speed controlis transferred from the first one of the plurality Of speed controlsystems to the second one of the speed control systems whilst the firstis accelerating the vehicle to the target speed of the first, the secondone of the speed control systems is operable to continue acceleratingthe vehicle at a rate corresponding to that of the first speed controlsystem.

The rate may be substantially equal to that applied by the first speedcontrol system.

It is so be understood that the second one of the speed control systemsmay continue accelerating the vehicle at substantially the same rateprovided the second one of the speed control systems determines that anincrease in vehicle speed above an instant speed of the vehicle isrequired. The second one of the speed control systems may accelerate thevehicle to the target speed thereof, which is set to be substantiallyequal to the target speed of the first speed control system, providedany temporary maximum speed imposed by the second one of the speedcontrol systems does not exceed the second target speed value.

In a further aspect of the invention for which protection is soughtthere is provided a vehicle comprising a control system according toanother aspect.

In a still further aspect of the invention for which protection issought there is provided a method of controlling a vehicle comprising:controlling a speed of a vehicle by means of one of a plurality of speedcontrol systems; and transferring responsibility from a first one of theplurality of speed control systems to a second one of the speed controlsystems, the method comprising setting a value of target speed of thesecond one of the speed control systems to a value dependent on thetarget speed of the first one of the speed control systems whenresponsibility for speed control is transferred from the first one ofthe plurality of speed control systems to the second.

The method may comprise setting the value of target speed of the secondone of the speed control systems to a value substantially equal to thetarget speed of the first one of the speed control systems.

In one aspect of the invention for which protection is sought there isprovided a vehicle control system comprising a plurality of speedcontrol systems each operable to cause the vehicle to operate inaccordance with a respective target speed, the system being operablewherein one of said plurality of speed control systems may be selectedto control vehicle speed at a given moment in time,

wherein when responsibility for speed control is transferred from afirst one of the plurality of speed control systems to a second one ofthe speed control systems, the second one of the speed control systemsis operable to set a value of target speed thereof to a valuecorresponding to that of the target speed of the first one of the speedcontrol systems.

It is to be understood that the set-speed may also be referred to hereinas a “target speed” and the terms “target speed” and ‘set-speed’ areused interchangeably herein.

Within the scope of this application it is expressly intended that thevarious aspects, embodiments, examples aid alternatives set out in thepreceding paragraphs, in the claims and/or in the following descriptionand drawings, and in particular the individual features thereof, may betaken independently or in any combination. Features described inconnection with one embodiment are applicable to all embodiments, unlesssuch features are incompatible.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described by way ofexample only with reference to the following figures in which:

FIG. 1 is a schematic illustration of a vehicle according to anembodiment of the invention in plan view;

FIG. 2 shows the vehicle of FIG. 1 in side view:

FIG. 3 is a high level schematic diagram of an embodiment of the vehiclespeed control system of the present invention, including a cruisecontrol system and a low-speed progress control system;

FIG. 4 is a schematic diagram of further features of the vehicle speedcontrol system in FIG. 3;

FIG. 5 illustrates, a steering wheel and brake and accelerator pedals ofa vehicle according to an embodiment of the present invention;

FIG. 6 Illustrates a speedometer of the embodiment of FIG. 1; and

FIG. 7 is a flow diagram illustrating operation of a vehicle controlsystem according to an embodiment of the present invention.

DETAILED DESCRIPTION

References herein to a block such as a function block are to beunderstood to include reference to software code for performing thefunction or action specified which may be an output that is providedresponsive to one or more inputs. The code may be in the form of asoftware routine or function called by a main computer program, or maybe code forming part of a flow of code not being a separate routine orfunction. Reference to function block is made for ease of explanation ofthe manner of operation of embodiments of the present invention.

FIG. 1 shows a vehicle 100 according to an embodiment of the presentinvention. The vehicle 100 has a powertrain 129 that includes an engine121 that is connected to a driveline 130 having an automatictransmission 124. It is to be understood that embodiments of the presentinvention are also suitable for use in vehicles with manualtransmissions, continuously variable transmissions or any other suitabletransmission.

The driveline 130 is arranged to drive a pair of front vehicle wheels111,112 by means of a front differential 137 and a pair of front driveshafts 118. The driveline 130 also comprises an auxiliary drivelineportion 131 arranged to drive a pair of rear wheels 114, 115 by means ofan auxiliary driveshaft or prop-shaft 132, a rear differential 135 and apair of rear driveshafts 139. Embodiments of the invention are suitablefor use with vehicles in which the transmission is arranged to driveonly a pair of front wheels or only a pair of rear wheels (i.e. frontwheel drive vehicles or rear wheel drive vehicles) or selectable twowheel drive/four wheel drive vehicles. In the embodiment of FIG. 1 thetransmission 124 is releasably connectable to the auxiliary drivelineportion 131 by moans of a power transfer unit (PTU) 131P, allowingselectable two wheel drive or four wheel drive operation. It is to beunderstood that embodiments of the invention may be suitable forvehicles having more than four wheels or where only two wheels aredriven, for example two wheels of a three wheeled vehicle or fourwheeled vehicle or a vehicle with mere than four wheels.

A control system for the vehicle engine 121 includes a centralcontroller 10, referred to as a vehicle control unit (VCU) 10, apowertrain controller 11, a brake controller 13 and a steeringcontroller 170C. The brake controller 13 forms part of a braking system22 (FIG. 3). The VCU 10 receives and outputs a plurality of signals toand from various sensors and subsystems (not shown) provided on thevehicle. The VCU 10 includes a low-speed progress (LSP) control system12 shown in FIG. 3 and a stability control system (SCS) 14. The SCS 14Improves the safety of the vehicle 100 by defecting and managing loss oftraction. When a reduction in traction or steering control is detected,the SCS 14 is operable automatically to command a brake controller 13 toapply one or more brakes of the vehicle to help to steer the vehicle 100in the direction the user wishes to travel. In the embodiment shown theSCS 14 is implemented by the VCU 10. In some alternative embodiments theSCS 14 may be implemented by the brake controller 13. Furtheralternatively, the SCS 14 may be implemented by a separate controller.

Although not shown in detail in FIG. 3, the VCU 10 further includes aDynamic Stability Control (DSC) function block, a Traction Control (TC)function block, an Anti-Lock Braking System (ABS) function block and aHill Descent Control (HDC) function block. These function blocks areimplemented in software code run by a computing device of the VCU 10 andprovide outputs indicative on for example, DSC activity, TC activity,ABS activity, brake interventions on individual wheels and engine torquerequests from the VCU 10 to the engine 121 in the event a wheel slipevent occurs. Each of the aforementioned events indicate that a wheelslip event has occurred. Other vehicle sub-systems such as a rollstability control system or the like may also be useful.

As noted above the vehicle 100 also includes a cruise control system 16which is operable to automatically maintain vehicle speed at a selectedspeed when the vehicle is travelling at speeds in excess of 25 kph. Thecruise control system 16 is provided with a cruise control HMI (humanmachine interface) 18 by which means the user can input a target vehiclespeed to the cruise control system 16 in a known manner. In oneembodiment of the invention, cruise control system input controls aremounted to a steering wheel 171 (FIG. 5). The cruise control system 16may be switched on by pressing a cruise control system selector button176. When the cruise control system 16 is switched on, depression of a‘set-speed’ control 173 sets the current value of a cruise controlset-speed parameter, cruise_set-speed to the current vehicle speed.Depression of a ‘+’ button 174 allows the value of cruise_set-speed tobe increased whilst depression of a ‘−’ button 175 allows the value ofcruise_set-speed to be decreased. A resume button 173R is provided thatis operable to control the cruise control system 16 to resume speedcontrol at the instant value of cruise_set-speed following driverover-ride. It is to be understood that known on-highway cruise controlsystems including the present system 16 are configured so that, in theevent that the user depresses a brake pedal 163 or, in the case ofvehicles with a manual transmission, a clutch pedal, the cruise controlfunction is cancelled and the vehicle 100 reverts to a manual mode ofoperation which requires input from an accelerator pedal 161 by a userin order to maintain vehicle speed. In addition, defection of a wheelslip event, as may be initiated by a loss of traction, also has theeffect of cancelling the cruise control function. Speed control by thesystem 16 is resumed if the driver subsequently depresses the resumebutton 173R.

The cruise control system 16 monitors vehicle speed and any deviationfrom the target vehicle speed is adjusted automatically so that thevehicle speed is maintained at a substantially constant value, typicallyin excess of 25 kph. In other words, the cruise control system isineffective at speeds lower than 25 kph. The cruise control HMI 18 mayalso be configured to provide an alert to the user about the status ofthe cruise control system 16 via a visual display of the HMI 18. In thepresent embodiment the cruise control system 16 is configured to allowthe value of cruise_set-speed to be set to any value in the range 25-150kph.

The LSP control system 12 also provides a speed-based control system forthe user which enables the user to select a very low target speed atwhich the vehicle can progress without any pedal inputs being requiredby the user. Low-speed speed control (or progress control) functionalityis not provided by the on-highway cruise control system 16 whichoperates only at speeds above 25 kph.

The LSP control system 12 is activated by means of a LSP control systemselector button 172 mounted on the steering wheel 171. The system 12 isoperable to apply selective powertrain, traction control and brakingactions to one or more wheels of the vehicle 100, collectively orindividually, to maintain the vehicle 100 at the desired speed.

The LSP control system 12 is configured to allow a user to input adesired value of set-speed parameter, LSP_set-speed to the LSP controlsystem 12 via a low-speed progress control HMI (LSP HMI) 20 (FIG. 1,FIG. 3) which shares certain input buttons 173-175 with the cruisecontrol system 16 and HDC control system 12HD. Provided the vehiclespeed is within the allowable range of operation of the LSP controlsystem (which is the range from 2 to 30 kph in the present embodimentalthough other ranges are also useful) the LSP control system 12controls vehicle speed in accordance with the value of LSP_set-speed.Unlike the cruise control system 16, the LSP control system 12 isconfigured to operate independently of the occurrence of a tractionevent. That is, the LSP control system 12 does not cancel speed controlupon detection of wheel slip. Rather, the LSP control system 12 activelymanages vehicle behaviour when slip is defected.

The LSP HMI 20 is provided in the vehicle cabin so as to be readilyaccessible to the user. The user of the vehicle 100 is able to input tothe LSP control system 12, via the LSP HMI 20, an indication of thespeed at which the user desires the vehicle to travel (referred to as“the target speed”) by means of the ‘set-speed’ button 173 and the‘+’/‘−’ buttons 174, 175 in a similar manner to the cruise controlsystem 16. The LSP HMI 20 also includes a visual display upon whichinformation and guidance can be provided to the user about the status ofthe LSP control system 12.

The LSP control system 12 receives an input from the braking system 22of the vehicle indicative of the extent to which the user has appliedbraking by means of the brake pedal 163. The LSP control system 12 alsoreceives an input from an accelerator pedal 161 indicative of the extentto which the user has depressed the accelerator pedal 161. An input isalso provided to the LSP control system 12 from the transmission orgearbox 124. This input may include signals representative of, forexample, the speed of an output shaft of the gearbox 124, torqueconverter slip and a gear ratio request. Other inputs to the LSP controlsystem 12 include an input from the cruise control HMI 18 which isrepresentative of the status (ON/OFF) of the cruise control system 16,and an input from the LSP control HMI 20.

The HDC function block of the VCU 10 forms part of a HDC system 12HD.When the HDC system 12HD is active, the system 12HD controls the brakingsystem 22 (of which the ABS function block forms part) in order to limitvehicle speed to a value corresponding to that of a HDC set-speedparameter HDC_set-speed which may be set by a user. The HDC set-speedmay also be referred to as an HDC target speed. Provided the user doesnot override the HDC system by depressing the accelerator pedal 161 whenthe HDC system is active, the HDC system 12HD controls the brakingsystem 22 (FIG. 3) to prevent vehicle speed from exceeding theHDC_set-speed. In the present embodiment the HDC system 12HD is notoperable to apply positive drive torque. Rather, the HDC system 12HD isonly operable to apply negative brake torque.

A HDC system HMI 20HD is provided by means of which a user may controlthe HDC system 12HD, including setting the value of HDC_set-speed. AnHDC system selector button 177 is provided on the steering wheel 171 bymeans of which a user may activate the HDC system 12HD to controlvehicle speed.

As noted above, the HDC system 12HD is operable to allow a user to set avalue of HDC set-speed parameter HDC_set-speed and to adjust the valueof HDC_set-speed using the same controls as the cruise control system 16and LSP control system 12. Thus, in the present embodiment, when the HDCsystem 12HD is controlling vehicle speed, the HDC system set-speed maybe increased, decreased or set to an instant speed of the vehicle, thatis to say the prevailing speed of the vehicle, in a similar manner tothe set-speed of the cruise control system 16 and LSP control system,using the same control buttons 173, 173R, 174, 175. The HDC system 12HDis operable to allow the value of HDC_set-speed to be set to any valuein the range from 2-30 kph. Other values are also useful.

If the HDC system 12HD is selected when the vehicle 100 is travelling ata speed of 30 kph or less and no other speed control system is inoperation, the HDC system 12HD assumes a standby mode. If the usersubsequently presses the ‘set-speed’ button 173 the HDC system 12HD setsthe value of HDC_set-speed to the instant or prevailing vehicle speed.If the HDC system 12HD is selected when the vehicle 100 is travelling ata speed above 30 kph but not exceeding 50 kph and the driver is notdepressing the accelerator pedal 161, the HDC system 12HD is configuredto slow the vehicle to 30 kph. being the maximum allowable value ofHDC_set-speed by means of the powertrain 129 and/or braking system 22 ata deceleration rate not exceeding a maximum allowable rate. The rate maybe 1.25 ms-2 or any other suitable value. The HDC system then assumesthe standby mode until the driver sets a value of HDC_set-speed.

It is to be understood that the VCU 10 is configured to implement aknown Terrain Response (TR) (RIM) System of the kind described above inwhich the VCU 10 controls settings of one or more vehicle systems orsub-systems such as the powertrain controller 11 in dependence on aselected driving mode. The driving mode may be selected by a user bymeans of a driving mode selector 141S (FIG. 1). The driving modes mayalso be referred to as terrain modes, terrain response modes, terrainprograms or control modes. In the embodiment of FIG. 1 four drivingmodes are provided: an ‘on-highway’ driving mode suitable for driving ona relatively hard, smooth driving surface where a relatively highsurface coefficient of friction exists between the driving surface andwheels of the vehicle; a ‘sand’ driving mode suitable for driving oversandy terrain: a ‘grass, gravel or snow’ driving mode suitable fordriving over grass, gravel or snow, a ‘rock crawl’ driving mode suitablefor driving slowly over a rocky surface; and a ‘mud and ruts’ drivingmode suitable for driving in muddy, rutted terrain. Other driving modesmay be provided in addition or instead.

In some embodiments, the LSP control system 12 may be in either one ofan active condition, a standby condition and an ‘off’ condition. In theactive condition, the LSP control system 12 actively manages vehiclespeed by controlling powertrain torque and braking system torque. In thestandby condition, the LSP control system 12 does not control vehiclespeed until a user presses the resume button 173R or the ‘set speed’button 173. In the off condition the LSP control system 12 is notresponsive to input controls unless the LSP control system selectorbutton 172 is depressed.

In the present embodiment the LSP control system 12 is also operable toassume an intermediate condition similar to that of the active mode butin which the LSP control system 12 is prevented from commanding theapplication of positive drive torque to one or more wheels of thevehicle 100 by the powertrain 129. Thus, only braking torque may beapplied, by means of the braking system 22 and/or powertrain 129. Thusthe system 12 may prevent excessive speeds from being attained duringdescent. The intermediate mode may be referred to as a LSP-hill descent(or LSP-HD) mode since it functions in substantially the same way as theHDC system 12HD. In some embodiments, the LSP control system 12 isconfigured to trigger the HDC system 12HD to control vehicle speed whenthe LSP control system 12 assumes the intermediate mode Otherarrangements are also useful.

With the LSP control system 12 in the active condition, the user mayincrease or decrease the vehicle set-speed by means of the ‘+’ and ‘−’buttons 174, 175. In addition, the user may also increase or decreasethe vehicle set-speed by lightly pressing the accelerator or brakepedals 101, 163 respectively. In some embodiments, with the LSP controlsystem 12 in the active condition the ‘+’ and ‘−’ buttons 174, 175 aredisabled such that adjustment of the value of LSP_set-speed can only bemade by means of the accelerator and brake pedals 161,163. This latterfeature may prevent unintentional changes in set-speed from occurring,for example due to accidental pressing of one of the ‘+’ or ‘−’ buttons174, 175. Accidental pressing may occur for example when negotiatingdifficult terrain where relatively large and frequent changes insteering angle may be required. Other arrangements are also useful.

It is to be understood that in the present embodiment the LSP controlsystem 12 is operable to cause the vehicle to travel in accordance witha value of set-speed in the range from 2-30 kph whilst the cruisecontrol system is operable to cause the vehicle to travel in accordancewith a value of set-speed in the range from 25-150 kph although othervalues are also useful. If the LSP control system 12 is selected whenthe vehicle speed is above 30 kph but less than or substantially equalto 50 kph, the LSP control system 12 assumes the intermediate mode inwhich the braking system 22 is employed to slow the vehicle 100 to avalue of set-speed corresponding to the value of parameterLSP_set-speed. Once the vehicle speed falls to 30 kph or below, the LSPcontrol system 12 assumes the active condition in which it is operableto apply positive drive torque via the powertrain 129, as well as braketorque via the powertrain 129 (via engine braking) and the brakingsystem 22 in order to control the vehicle in accordance with theLSP_set-speed value. If no LSP set-speed value has been set, the LSPcontrol system 12 assumes the standby mode.

It is to be understood that if the LSP control system 12 is in theactive mode, operation of the cruise control system 16 is inhibited. Thetwo systems 12, 16 therefore operate independently of one another, sothat only one can be operable at any one time, depending on the speed atwhich the vehicle is travelling.

In some embodiments, the cruise control HMI 18 and the LSP control HMI20 may be configured within the same hardware so that, for example, thespeed selection is input via the same hardware, with one or moreseparate switches being provided to switch between the LSP input and thecruise control input.

FIG. 4 illustrates the means by which vehicle speed is controlled in theLSP control system 12. As described above, a speed selected by a user(set-speed) is input to the LSP control system 12 via the LSP controlHMI 20. A vehicle speed sensor 34 associated with the powertrain 129(shown in FIG. 1) provides a signal 36 indicative of vehicle speed tothe LSP control system 12. The LSP control system 12 includes acomparator 28 which compares the set-speed 38 (also referred to as a‘target speed’ 38) selected by the user with the measured speed 36 andprovides an output signal 30 indicative of the comparison. The outputsignal 30 is provided to an evaluator unit 40 of the VCU 10 whichinterprets the output signal 30 as either a demand for additional torqueto be applied to the vehicle wheels 111-115, or for a reduction intorque applied to the vehicle wheels 111-115, depending on whether thevehicle speed needs to be increased or decreased to maintain the speedLSP_set--speed. An increase in torque is generally accomplished byincreasing the amount of powertrain torque delivered to a given positionof the powertrain, for example an engine output shaft, a wheel or anyother suitable location. A decrease in torque at a given wheel to avalue that is less positive or more negative may be accomplished bydecreasing powertrain torque delivered to a wheel and/or by increasing abraking force on a wheel. It is to be understood that in someembodiments in which a powertrain 129 has one or more electric machinesoperable as a generator, negative torque may be applied by thepowertrain 129 to one or more wheels by the electric machine. Negativetorque may also be applied by means of engine braking in somecircumstances, depending at least in part on the speed at which thevehicle 100 is moving. If one or more electric machines are providedthat are operable as propulsion motors, positive drive torque may beapplied by means of the one or more electric machines.

An output 42 from the evaluates unit 40 is provided to the powertraincontroller 11 and brake controller 13 which in turn control a net torqueapplied to the vehicle wheels 111-115. The net torque may be increasedor decreased depending on whether the evaluator unit 40 demands positiveor negative torque. In order to cause application of the necessarypositive or negative torque to the wheels, the evaluator unit 40 maycommand that positive or negative torque is applied to the vehiclewheels by the powertrain 129 and/or that a braking force is applied tothe vehicle wheels by the braking system 22, either or both of which maybe used to implement the change in torque that is necessary to attainand maintain a required vehicle speed. In the illustrated embodiment thetorque is applied to the vehicle wheels individually so as to maintainthe vehicle at the required speed, but in another embodiment torque maybe applied to the wheels collectively to maintain the required speed. Insome embodiments, the powertrain controller 11 may be operable tocontrol an amount of torque applied to one or more wheels by controllinga driveline component such as a rear drive unit, front drive unit,differential or any other suitable component. For example, one or morecomponents of the driveline 130 may include one or more clutchesoperable to allow an amount of torque applied to one or more wheels tobe varied. Other arrangements are also useful. Where a powertrain 129includes one or more electric machines, for example one or morepropulsion motors and/or generators, the powertrain controller 11 may beoperable to modulate torque applied to one or more wheels by means ofone or more electric machines.

The LSP control system 12 also receives a signal 48 indicative of awheel slip event having occurred. This may be the same signal 48 that issupplied to the on-highway cruise control system 16 of the vehicle, andwhich in the case of the latter triggers an override or inhibit mode ofoperation in the on-highway cruise control system 16 so that automaticcontrol of vehicle speed by the on-highway cruise control system 16 issuspended or cancelled. However, the LSP control system 12 is notarranged to cancel or suspend operation in dependence on receipt of awheel slip signal 48 indicative of wheel slip. Rather, the system 12 isarranged to monitor and subsequently manage wheel slip so as to reducedriver workload. During a slip event, the LSP control system 12continues to compare the measured vehicle speed with the value ofLSP_set-speed, and continues to control automatically the torque appliedto the vehicle wheels so as to maintain vehicle speed at the selectedvalue. It is to be understood therefore that the LSP control system 12is configured differently to the cruise control system 16, for which awheel slip event has the effect of overriding the cruise controlfunction so that manual operation of the vehicle must be resumed, orspeed control by the cruise control system 12 resumed by pressing theresume button 173R or set-speed button 173.

In a further embodiment of the present invention (not shown) a wheelslip signal 48 is derived not just from a comparison of wheel speeds,but further refined using sensor data indicative of the vehicle's speedover ground. Such a speed over ground determination may be made viaglobal positioning (GPS) data, or via a vehicle mounted radar or laserbased system arranged to determine the relative movement of the vehicle100 and the ground over which if is travelling. A camera system may beemployed for determining speed over ground in some embodiments.

At any stage of the LSP control process the user can override thefunction by depressing the accelerator pedal 161 and/or brake pedal 163to adjust the vehicle speed in a positive or negative sense. However, inthe event that a wheel slip event is determined via signal 48, the LSPcontrol system 12 remains active and control of vehicle speed by the LSPcontrol system 12 is not suspended. As shown in FIG. 4, this may beimplemented by providing a wheel slip event signal 48 to the LSP controlsystem 12 which is then managed by the LSP control system 12. In theembodiment shown in FIG. 1 the SCS 14 generates the wheel slip eventsignal 48 and supplies it to the LSP control system 12 and cruisecontrol system 16.

A wheel slip event is triggered when a loss of traction occurs at anyone of the vehicle wheels. Wheels and tyres may be more prone to losingtraction when travelling for example on snow, ice, mud or sand and/or onsteep gradients or cross-slopes. A vehicle 100 may also be more prone tolosing traction in environments where the terrain is more uneven orslippery compared with driving on a highway in normal on-roadconditions. Embodiments of the present invention therefore findparticular benefit when the vehicle 100 is being driven in an off-roadenvironment, or in conditions in which wheel slip may commonly occur.Manual operation by the user in such conditions can be a difficult andoften stressful experience and may result in an uncomfortable ride forvehicle occupants.

The vehicle 100 is also provided with additional sensors (not shown)which are responsive to a variety of different parameters associatedwith vehicle motion and status. These may be inertial systems unique tothe LSP or HDC control system 12, 12HD or part of an occupant restraintsystem or any other sub-system which may provide data from sensors suchas gyres and/or accelerometers that may be indicative of vehicle bodymovement and may provide a useful input to the LSP and/or HDC controlsystems 12, 12HD. The signals from the sensors provide, or are used tocalculate, a plurality of driving condition indicators (also referred toas terrain indicators) which are indicative of the nature of the terrainconditions over which the vehicle is travelling.

The sensors (not shown) on the vehicle 100 include, but ere not limitedto, sensors which provide continuous sensor outputs to the VCU 10,including wheel speed sensors, as mentioned previously and as shown inFIG. 5, an ambient temperature sensor, an atmospheric pressure sensor,tyre pressure sensors, wheel articulation sensors, gyroscopic sensors todetect vehicular yaw, roll and pitch angle and rate, a vehicle speedsensor, a longitudinal acceleration sensor, an engine torque sensor (orengine torque estimator), a steering angle sensor, a steering wheelspeed sensor, a gradient sensor (or gradient estimator), a lateralacceleration sensor which may be part of the stability control system(SCS), a brake pedal position sensor, a brake pressure sensor, anaccelerator pedal position sensor, longitudinal, lateral and verticalmotion sensors, and wafer detection sensors forming part of a vehiclewading assistance system (not shown). In other embodiments, only aselection of the aforementioned sensors may be used.

The VCU 10 also receives a signal from the steering controller 170C. Thesteering controller 170C is in the form of an electronic power assistedsteering unit (ePAS unit). The steering controller 170C provides asignal to the VCU 10 indicative of the steering force being applied tosteerable road wheels 111, 112 of the vehicle 100. This forcecorresponds to that applied by a user to the steering wheel 171 incombination with steering force generated by the ePAS unit 170C.

The VCU 10 evaluates the various sensor inputs to determine theprobability that each of a plurality of different control modes (drivingmodes) for the vehicle subsystems is appropriate, with each central modecorresponding to a particular terrain type over which the vehicle istraveling (for example, mud and ruts, sand, grass/gravel/snow).

If the user has selected operation of the vehicle in an automaticdriving mode selection condition, the VCU 10 then selects the mostappropriate one of the control modes and is configured automatically tocontrol the subsystems according to the selected mode. This aspect ofthe invention is described in further detail in our co-pending patentapplication nos. GB1111288.5, GB1211910.3 and GB1202427.9, the contentsof each of which is incorporated herein by reference.

The nature of the terrain over which the vehicle is travelling (asdetermined by reference to the selected control mode) may also beutilised in the LSP control system 12 to determine an appropriateincrease or decrease: in drive torque that is to be applied to thevehicle wheels. For example, if the user selects a value ofLSP_set-speed that is not suitable for the nature of the terrain overwhich the vehicle is travelling, the system 12 is operable toautomatically adjust the vehicle speed downwards by reducing the speedof the vehicle wheels. In some cases, for example, the user selectedspeed may not be achievable or appropriate over certain terrain types,particularly in the case of uneven or rough surfaces. If the system 12selects a set-speed that differs from the user-selected set-speed, avisual indication of the speed constraint is provided to the user viathe LSP HMI 20 to indicate that an alternative speed has been adopted.

As described above, the vehicle 100 has three speed control systems: alow speed progress control (LSP) system 12, an on-highway cruise controlsystem 16 and a hill descent control (HDC) system 12HD. Each of thesystems is configured to store in a memory a current value of acorresponding set-speed parameter associated with that system.

The speed control systems 12, 16, 12HD are operable to communicate withone another via control lines connected therebetween in order to ‘share’their respective set-speed values with one another. In some embodimentsthe systems 12, 16, 12HD are configured to communicate with one anothervia a controller area network (CAN) bus, by means of a private network,or by any other suitable means. In some embodiments the control systems12, 16, 12HD are implemented in software code run on a single computingdevice that stores data in respect of set-speeds of the respectivecontrol systems in a memory. Other arrangements are also useful

When operating with any one of the control systems 12, 16, 12HD in an‘active’ state in which the system 12, 16, 12HD is responsible forcontrolling vehicle speed, the user may increase or decrease set speedby means of the ‘+’ and ‘−’ buttons 174, 175 or cause the set-speed tobe set to the current vehicle speed by depressing the ‘set-speed’control button 173.

A user may switch from one speed control system (being the ‘currentlyselected’ speed control system) to another speed control system bysimply selecting the required speed control system by means of thecorresponding selector control 172, 176, 177.

When the user does this, the ‘newly selected’ speed control systemchecks the Instant value of vehicle speed. If the instant value ofvehicle speed is outside a range of speed values permitting operation ofthe newly selected speed control system, the VCU 10 ignores the userrequest to switch to a different speed control system and the currentlyselected speed control system remains in control of vehicle speed.Notification may be provided to the user indicating the reason the VCU10 has ignored the request.

If the instant speed of the vehicle 100 is within the range of allowablevalues for operation of the newly selected speed control system, the VCU10 checks whether the value of set-speed parameter of the currentlyselected speed control system falls within the range of allowable valuesof the newly selected system. If the value does fall within the range ofallowable values, the newly selected system sets the value of set-speedparameter thereof to the value of set-speed parameter of the currentlyselected system. The currently selected speed control system is thendeselected and the newly selected speed control system assumes controlof vehicle speed. The newly selected speed control system then controlsvehicle speed in accordance with the value of set-speed parameterthereof.

In some embodiments, if a user selects a new speed control system bymeans of the corresponding selector control 172, 176, 177 when thecurrent vehicle speed is not within the range of allowable speeds forwhich the newly selected speed control system may become active, and theuser either holds the selector control in a ‘select’ condition forperiod exceeding a prescribed period (such as 1 s) or selects the newspeed control system more than once within a period of (say) 1 s (or anyother suitable value), the VCU 10 is configured to command a change invehicle speed to bring the vehicle speed within the range of allowablevalues of speed for the newly selected speed control system.

If, before the newly selected speed control system takes over speedcontrol, the newly selected speed control system determines that thevalue of set-speed parameter of the currently selected speed controlsystem is not within the permitted range of values of speed of the newlyselected system, but the instant vehicle speed is within a range ofvalues for which the newly selected system is permitted to assume anintermediate mode in which the newly selected system cannot commandapplication of positive drive torque but may command application ofbrake torque (e.g. by means of the powertrain 129 and/or braking system22), the value of set-speed of the newly selected speed control systemis set to a value that is the closest allowable value to that of thecurrently selected speed control system. The newly selected speedcontrol system then assumes the intermediate mode of operation and thepreviously selected speed control system is deselected. In thissituation, once vehicle speed subsequently falls within the range ofallowable values of the newly selected speed control system in theactive condition (or mode), the newly selected speed control systemassumes the active condition (or mode) and takes over control of vehiclespeed in accordance with the value of set-speed parameter of that speedcontrol system.

As stated above if instant vehicle speed is outside the range ofpermitted values for operation of a newly selected speed control systemin either its active or any intermediate mode of operation, thecurrently selected speed control system may continue to control vehiclespeed. An indication may be provided to a user informing the user thatthe newly selected speed control system is inoperable at the currentvehicle speed. Thus, a new speed control system may only be selectablefor vehicle speed control when the current speed of the vehicle 100falls within the allowable range of speeds of an active or intermediatemode of operation of the new speed control system. Other arrangementsare also useful.

By way of example, in use the vehicle 100 may be travelling on-highwaywith the cruise control system 16 active and responsible for maintainingvehicle speed in accordance with a current value of cruise_set-speedparameter. The driver may then depress the LSP control system selectorbutton 172 to switch speed control from the cruise control system 16 tothe LSP control system 12.

In this situation, the LSP control system 12 checks whether instantvehicle speed is within the allowable range for operation of the LSPcontrol system 12 in the intermediate or active modes (2-50 kph). If theinstant speed is within the allowable range, the system 12 checks thecurrent value of cruise_set-speed parameter. If the value ofcruise_set-speed parameter is within the range of allowable values ofLSP_set-speed, i.e. in the range from 2-50 kph, the LSP control system12 sets the value of LSP_set-speed to the current value ofcruise_set-speed. The cruise control system 16 is then deselected andthe LSP control system 12 is selected for operation in the intermediateor active mode depending on the vehicle speed.

If the value of cruise_set-speed is greater than the maximum allowablevalue of LSP_set-speed, but the vehicle speed is within the range ofoperation of the LSP control system in the intermediate or active modes,the LSP control system 12 sets the value of LSP_set-speed to the instantvehicle speed if the instant speed is within the range of allowablevalues of LSP_set-speed. If the instant speed is above the maximumallowable value of LSP_set-speed, the LSP control system 12 sets thevalue of LSP_set-speed to the maximum allowable value of LSP_set-speed(being 30 kph in the present embodiment). The cruise control 16 is thendeselected and the LSP control system 12 is selected for operation inthe intermediate or active mode depending on the vehicle speed.

As described above, when in the intermediate mode of operation the LSPcontrol system 12 is configured to slow the vehicle to the LSP_set-speedvalue using the braking system 22 and powertrain 129. However the LSPcontrol system 12 is not permitted to apply positive drive torque(positive powertrain torque) when in the intermediate mode. It is to beunderstood that the intermediate mode of operation of the LSP controlsystem 12 is similar to the active mode of operation of the HDC system12HD. That is, the powertrain 129 and braking system 22 may be employedto slow the vehicle, but the HDC system 12HD is not permitted to commandpositive powertrain torque.

If the vehicle speed is such that the intermediate mode of operation ofthe LSP control system 12 is assumed, the LSP control system 12 employsthe powertrain 129 and braking system 22 as required in order to slowthe vehicle 100 to the LSP_set-speed. Once the vehicle speed is withinthe allowable range of operation of the LSP control system 12 in theactive mode (i.e. in the range from 2-30 kph) the LSP control system 12assumes the active mode and attempts to control vehicle speed inaccordance with the LSP_set-speed value. If the VCU 10 has determinedthat the value of LSP_set-speed is too high for the prevailing terrainconditions, the LSP control system 12 may be commanded temporarily tocontrol the vehicle in accordance with a reduced value of set-speed.

If when the cruise control system 16 is the currently selected speedcontrol system the driver depresses the LSP control system selectorbutton 172 when the instant or prevailing vehicle speed is within theallowable range of values for the LSP control system 12 in the activemode and the value of cruise_set-speed is within the allowable range ofvalues of LSP_set-speed, the value of LSP_set-speed is set to besubstantially equal to the currant value of cruise_set-speed. The cruisecontrol system 16 is then deselected and the LSP control system 12selected as the system responsible for speed control in the active modeof operation.

If the driver presses the resume button 173R for the first time alterselecting a new speed control system, and the new speed control systemis in an active or intermediate mode or condition, the value ofset-speed of the newly selected speed control system is set to the mostrecent value of set-speed that was employed by that control system thelast time that control system was active.

Thus, if the driver presses the resume button 173R for the first timeafter selecting the LSP control system 12 and the LSP control system 12has assumed one of the active or intermediate modes, the value ofLSP_set-speed is set to the most recent value of LSP_set-speed that wasemployed by the LSP control system 12 the last time the LSP controlsystem 12 was active. It is to be understood that this functionality isalso provided for the HDC control system 12HD and the cruise controlsystem 16 although other arrangements are also useful. In someembodiments, a check may be performed to ensure that this most recent,previous value of set-speed is appropriate to the prevailing terrainconditions (e.g. by reference to the selected terrain mode, and/or adetermination whether the vehicle is wading) before allowing the valueof LS_set-speed to be changed.

FIG. 6 is a schematic illustration of a speedometer 190S of the vehicle100 that is visible to the driver whilst driving the vehicle 100. Thespeedometer has a needle 191N that points to speed indicia on asubstantially circular dial 191D thereby to indicate current vehiclespeed.

When either the LSP control system 12 or the HDC system 12HD is active,the range of speed over which the system 12, 12HD is operable to set avalue of set-speed thereof is highlighted on the speedometer 190S in theform of a band 192. This allows a driver to determine readily whetherthe current vehicle speed as indicated by the needle 191N is within arange of allowable values of set-speed. In the example shown, the LSPcontrol system 12 is active and the band 192 highlights the speed rangefrom 2 to 30 kph.

A speed indicator icon or chaplet 193C is displayed on an outer edge ofthe speedometer dial 191D and indicates the current value ofLSP_set-speed. A similar chaplet is displayed when the cruise controlsystem 16 or the HDC control system 12HD is active, indicating thecurrently set values of cruise_set-speed and HDC_set-speed respectively,

A ghost chaplet 193GC may also be displayed on the speedometer 191S. Thechest chaplet 193GC is displayed at a location corresponding to thevalue of set-speed that was employed when the currently selected speedcontrol system was previously selected to control vehicle speed. Thus,if the driver switches from the LSP control system 12 to another system12HD, 16 such as the HDC control system 12HD, the LSP control system 12stores the value of LSP_set-speed in a memory thereof and retains it foruse when the LSP control system 12 is next selected. When the LSPcontrol system 12 is next selected, the LSP control system 12 retrievesthe previously stored value of LSP_set-speed and displays the ghostchaplet 193GC at a location corresponding to the previously storedvalue. Other arrangements are also useful.

If the vehicle 100 is operating with the LSP control system 12 activeand the driver selects operation of the cruise control system 16 bydepressing the cruise control system selector button 176, the cruisecontrol system 16 determines whether the current value of LSP_set-speedis within the allowable range of values of cruise_set-speed (i.e.whether the value of LSP_set-speed is in the range from 25-30 kph, beingthe range of overlap of values of LSP_set-speed and cruise_set-speed),and whether vehicle speed is within the allowable range of values foroperation of the cruise control system 16 (i.e. 25-150 kph). If thevalue of LSP_set-speed is within the range 25-30 kph and vehicle speedis in the range from 25-150 kph, the cruise control system 16 sets thevalue of cruise_set-speed substantially equal to the current value ofLSP_set-speed. The LSP control system 12 is then deselected and thecruise control system 16 selected as the system responsible for speedcontrol in some embodiments, if the instant vehicle speed is greaterthan the current value of LSP_set-speed and vehicle speed is within therange of allowable values for the cruise control system 16 in the activemode, the value of cruise_set-speed is set to the instant value ofvehicle speed.

If the driver selects operation of the cruise control system 16 whenvehicle speed is within the range of allowable values for operation ofthe cruise control system 16 but the value of LSP_set-speed is below theminimum allowable value of cruise_set-speed, the LSP_control system 12is deactivated and the cruise control system 16 assumes a standby mode.If the user subsequently presses the set-speed button 173 the cruisecontrol system 16 assumes the active mode and sets the value ofcruise_set-speed to the instant value of vehicle speed in the usualmanner. If instead the driver presses the resume button 173R the cruisecontrol system 16 assumes the active mode and the value ofcruise_set-speed is set to the previously stored value ofcruise_set-speed. The stored value corresponds to the most recent valueof cruise_set-speed that was employed when the cruise control system 16was last active, as described above.

Embodiments of the present invention have the advantage that vehiclecomposure may be enhanced. This is because when a user switches from onespeed control system to another, the new speed control system does notsimply fake a ‘snapshot’ of actual vehicle speed at the moment it isselected and set the value of set-speed parameter of that system to thecurrent vehicle speed. Rather, the set-speed parameter of the new speedcontrol system is set to the value employed by the previous speedcontrol system.

The benefit of this feature may be appreciated by considering a scenarioin which a first speed control system is in the process of acceleratingthe vehicle from a speed below the set-speed value to the set-speedvalue when the driver selects a second different speed control systeminstead of the first. Subject to various conditions being met asdescribed above, the second speed control system will continueaccelerating the vehicle to the same set-speed value when it takes overresponsibility for speed control. Some embodiments of the presentinvention may therefore enhance vehicle composure by maintainingacceleration of the vehicle when the new speed control system assumesvehicle control. Furthermore, embodiments of the present invention mayreduce driver workload since selecting a new speed control system wouldotherwise necessitate selecting a new set-speed value separately fromthe set-speed employed in the previously selected speed control system.This requirement would add a further operational step to the driver'sworkload and potentially interrupt progress of the vehicle.

In one example of system operation, a vehicle 100 may be driving withthe LSP control system 12 active and controlling vehicle speed inaccordance with a value of LSP_set-speed of 30 kph. A driver of thevehicle 100 may depress the brake pedal 163 to slow the vehicle 100 to aspeed of 20 kph temporarily. The LSP control system 12 responds byassuming a standby mode in which it does not fake any action to controlthe braking system 22 or powertrain 129. When the driver subsequentlypresses the resume button 173R, the LSP control system 12 assumes theactive mode and begins to accelerate the vehicle 100 towards theLSP_set-speed value (30 kph in this example). If the user then selectsoperation of the cruise control system 16 the cruise control system 16checks that the current vehicle speed is within the allowable range ofoperation of the cruise control system 16. Provided the vehicle speedexceeds 25 kph when the cruise control system selector button 176 isdepressed, the cruise control system 16 verifies that the value ofLSP_set-speed is within the range of allowable values ofcruise_set-speed. Since a value of 30 kph is within the range from 25 to150 kph the cruise control system 16 then sets the value ofcruise_set-speed to the current value of LSP_set-speed. The LSP controlsystem 12 is then deselected and the cruise control system 16 selectedas the system responsible for speed control. The cruise control systemthen continues accelerating the vehicle 100 from its current speed tothe cruise_set-speed value (30 kph).

It is to be understood that if the driver selects operation of the HDCcontrol system 12HD when the LSP control system 12 or cruise controlsystem 16 is active, the HDC control system 12HD checks whether vehiclespeed is within the allowable range of values of speed of the HDC system12HD (i.e. 2-50 kph). If the speed is within the allowable range ofvalues, the system 12HD then checks whether the value of set-speedparameter for the currently selected speed control system is within theallowable range of values of HDC_set-speed (i.e. 2-30 kph). If thiscondition is also met, the HDC system 12HD sets the value ofHDC_set-speed to the value of set-speed of the currently selected speedcontrol system and assumes the active mode. The HDC system 12HD thencauses vehicle speed to be controlled in accordance with the value ofHDC_set-speed. That is, the HD system 12HD controls vehicle speed suchthat it does not exceed the value of HDC_set-speed.

In one example scenario, a user may operate the vehicle 100 with thecruise control system 16 active whilst driving on a highway with adesired value of cruise_set-speed of (say) 50 kph. The driver may reducethe value of cruise_set-speed down to a value of 25 kph as the vehicle100 approaches and then follows a farm track. As the driving surface ofthe farm track becomes increasingly rough, the driver may then selectthe LSP control system 12 by pressing the LSP control system selectorbutton 172. Because the value of cruise_set-speed (25 kph) is within therange of allowable values of the LSP control system 12 (i.e. 2-30 kph),and vehicle speed is within the range of allowable values for operationof the LSP control system 12 in the active mode, the LSP control system12 sets the value of LSP_set-speed to the value of cruise_set-speed,i.e. 25 kph, and assumes the active mode of operation.

As the track becomes more rough and/or slippery the driver may reducethe value of LSP_set-speed stiff further. The gradient of the track maythen become sufficiently steep that the driver decides to select the HDCcontrol system 12. The HDC control system 12 checks that the currentvehicle speed (say 10 kph) is within the allowable range for operationof the HDC control system (2-50 kph) and that the value of LSP_set-speedis within the allowable range of values of HDC_set-speed (2-30 kph). ifthese conditions are met, the HDC control system 12HD sets the value ofHDC_set-speed to the instant value of LSP_set-speed. The LSP controlsystem 12 is then deselected and the HDC control system 12HD assumescontrol of vehicle speed.

The user may then adjust the value of HDC_set-speed using the controlsmounted on the steering wheel 171.

If the driver subsequently reselects the LSP control system 12, the LSPcontrol system 12 checks whether vehicle speed is within the allowablerange of values of the LSP control system 12. If this is the case, theLSP_control-system checks that the current value of HDC_set-speed iswithin the range of allowable values of LSP_set-speed. If this is thecase, the value of LSP_set-speed is set to the instant value ofHDC_set-speed. The HDC control system 12HD is deselected and the LSPcontrol system 12 assumes speed control in the active or intermediatemodes depending on vehicle speed. As described above, the value ofLSP_set-speed employed when the LSP control system 12 was last in theactive or intermediate modes of operation is displayed on thespeedometer 190S by means of a ghost chaplet 193GC. The driver may setthe current value of LSP_set-speed to the value Indicated by the ghostchaplet 193GC by depressing the resume button 173R.

For example, if the vehicle is travelling under the control of the LSPcontrol system 12 with a set-speed LSP_set-speed of 25 km/h and a speedof substantially 25 km/h, and the driver selects the HDC control system12HD, the HDC control system 12HD sets the value of HDC_set-speed to 25km/h and assumes control of vehicle speed instead of the LSP controlsystem 12. The driver may then reduce the value of HDC_set-speed to(say) 10 km/h before switching control of vehicle speed back to the LSPcontrol system 12. The LSP control system 12 will then assume control ofvehicle speed, with the value of LSP_set-speed set to 10 km/h. If thedriver subsequently presses the resume button 173R, the LSP controlsystem 12 sets the value of LSP_set-speed to the value most recentlyemployed the previous time the LSP control system 12 was operating, i.e.25 km/h.

If the driver subsequently re-selects the HDC control system 12HD, thevalue of HDC_set-speed last employed by the HDC control system 12HD isdisplayed by the ghost chaplet 193GC, and the driver may set theprevailing value of HDC_set-speed when the HDC control system 12 isselected to the speed indicated by the ghost chaplet 193GC by pressingthe resume button 173R.

FIG. 7 is a flow diagram illustrating a method according to anembodiment of the present invention by which speed control by one system(system A) may be cancelled in favour of speed control by another system(system B).

At step S103 speed control system A is active. Speed control system Amay be any suitable speed control system such as a cruise control system16, LSP control system 12 or HDC control system 12HD.

At step S105, a controller such as VCU 10 determines whether a requestto switch to speed control system B has been received. Speed controlsystem B may be any other one of two or more speed control systems 16,12, 12HD that the vehicle 100 is provided with other than system A. Ifsuch a request has not been received the method continues at step S103.If such a request has been received the method continues at step S107.

At step S107 speed control system B checks whether a current value ofvehicle speed is within an allowable range for operation of system B. Ifthe speed is not within the allowable range, the method continues atstep S103. An indication may be provided to the user that operation ofsystem B is not permitted. If the speed is within the allowable range,the method continues at step S109.

At step S109 system B checks whether the set-speed value of system A iswithin the allowable range of values of system B. If set-speed is withinthe allowable range the method continues at step S111 and the value ifset-speed of system B is set to the value of set-speed of system A.

If at step S109 the value of set-speed of system A is determined is notto be within the allowable range of values of system B, the methodcontinues at step S113.

At step S113 the value of set-speed of system B is set to the closestallowable value to the instant value of set-speed of system A.

At step S115 speed control system A is deselected and speed controlsystem B is selected for vehicle speed control.

It is to be understood that when at step S115 speed control system B isselected, system B may be caused to assume one of a plurality of modesof operation. For example, if the set-speed of system B has been set toa value higher than that of system A and system B is capable of applyingpositive drive torque to accelerate the vehicle to a set-speed, system Bmay assume operation in a standby mode in which the system is notoperable to command application of positive drive torque. Otherarrangements are also useful.

It will be understood that the embodiments described above are given byway of example only and are not intended to limit the invention, thescope of which is defined in the appended claims.

Embodiments of the present invention may be understood by reference tothe following numbered paragraphs:

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, means “including but not limited to”, andis not intended to (and does not) exclude other moieties, additives,components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith.

This application claims priority from co-pending UK patent applicationno. GB1214651.0, the entire contents of which are expressly incorporatedherein by reference.

1. A vehicle control system comprising a plurality of speed controlsystems each operable to cause the vehicle to operate in accordance witha respective target speed, wherein when responsibility for speed controlis transferred from a first one of the plurality of speed controlsystems to a second one of the speed control systems, a value of targetspeed of the second speed control system is set to a value dependent onthe target speed of the first speed control system.
 2. A vehicle controlsystem according to claim 1 operable to set the target speed of thesecond one of the speed control systems to the instant value of thetarget speed of the first one of the speed control systems when thesecond one of the speed control systems assumes responsibility forcontrolling vehicle speed from the first one of the speed controlsystems.
 3. A vehicle control system according to claim 1 wherein one ormore of the speed control systems may be operable to cause the vehicleto operate in accordance with a respective temporary maximum speed valuethat is less than the instant target speed in dependence on one or moreparameters.
 4. A vehicle control system according to claim 3 wherein oneor more of the speed control systems are operable to resume operation atthe target speed thereof if the temporary maximum speed value becomesgreater than or equal to the target speed or the speed control system nolonger causes the vehicle to operate in accordance with the temporarymaximum speed value.
 5. A system according to claim 3 operable to setthe temporary maximum speed value of one or more of the speed controlsystems in dependence on a type of terrain in which the vehicle isoperating.
 6. A system according to claim 5 operable to receive one ormore signals indicative of a type of terrain in which the vehicle isoperating whereby the system may set the maximum allowable value oftarget speed.
 7. A vehicle control system according to claim 1 operableto store data in respect of a previous value of target speed for one ormore of the speed control systems, wherein if one said one or more speedcontrol systems is reselected to be responsible for speed controlfollowing deselection thereof, the reselected system is operable toresume operation at the value of target speed employed by the reselectedsystem when the system was last selected.
 8. A vehicle control systemaccording to claim 1 configured wherein if responsibility for speedcontrol is transferred from the first one of the plurality of speedcontrol systems to the second one of the speed control systems whilstthe first is accelerating the vehicle to the target speed of the first,the second one of the speed control systems is operable to continueaccelerating the vehicle at a rate corresponding to that of the firstspeed control system.
 9. A vehicle comprising a control system accordingto claim
 1. 10. A method of controlling a vehicle comprising:controlling a speed of a vehicle by means of one of a plurality of speedcontrol systems; and transferring responsibility from a first one of theplurality of speed control systems to a second one of the speed controlsystems, the method comprising setting a value of target speed of thesecond one of the speed control systems to a value dependent on thetarget speed of the first one of the speed control systems whenresponsibility for speed control is transferred from the first one ofthe plurality of speed control systems to the second.
 11. A methodaccording to claim 10 comprising setting the value of target speed ofthe second one of the speed control systems to a value substantiallyequal to the target speed of the first one of the speed control systems.12. (canceled)